The objectives of this study are to determine the stability of biological and antigenic characteristics of the California Group arboviruses which occur in Wisconsin. Current research has shown that plaque variation occurs in both LaCrosse (LAC) and Jamestown Canyon (JC) viruses. Passage of LAC through chipmunks and passage of JC through deer selects for large plaque subpopulations. Passage of LAC through gray squirrels, however, rapidly selects for smaller plaques; large plaque variants are eliminated by the second squirrel passage. Attempts to isolate large and small plaque variants from LAC and from JC viruses are in progress. The small plaque JC virus has been isolated and propagated in pure form. Future work will center about purification of these variants and assay of their biologic and antigenic properties. Although not in pure form, predominantly large plaque chipmunk origin LAC virus appears to be antigenically different from the small plaque type squirrel passage variant. Both originated from the prototype virus which produces a range of plaque sizes. An attempt will be made to devise a measure of neurovirulence, probably in laboratory mice, so that the pathogenicity of the variant subpopulations can be compared. These variants will also be tested for their ability to infect and be transmitted by arthropod vectors, to determine if selection by the vector is likely to occur.